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Most of us on this site believe that quantum computing will work. However, let's play devil's advocate. Imagine that we suddenly hit some fundamental stumbling block that prevented further development towards a universal quantum computer. Perhaps we're limited to a NISQ device (Noisy, Intermediate Scale Quantum) of 50-200 qubits, for the sake of argument. The study of (experimental) quantum computing suddenly stops and no further progress is made.

What good has already come out of the study of quantum computers?

By this, I mean realisable quantum technologies, the most obvious candidate being Quantum Key Distribution, but also technical results that feed into other fields. Rather than simply a list of items, a brief description of each would be appreciated.

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    $\begingroup$ While this question was clearly asked with some additional background motivation, it has undoubtedly been one of the most successful questions on this site so far, so I wanted to try and ask something that heads in a similar direction, but without the hidden agenda. $\endgroup$ – DaftWullie Jul 4 '18 at 7:12
  • $\begingroup$ You are mentioning Universal Quantum Computer and Quantum Key Distribution in the same question but my understanding is that Quantum Key Distribution is just a secure communication method between 2 points which is not really related to a universal quantum computer apart from the fact that both are based on quantum mechanics. $\endgroup$ – JanVdA Jan 1 at 23:16
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    $\begingroup$ Don't have time for a long answer but quantum-inspired classical algorithms are making some serious advances. See the work of E Tang and Katzgraber. $\endgroup$ – Andrew O Jan 2 at 23:13
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There are a lot of interesting applications that use similar technology. A lot of labs that work towards quantum computing also publish papers with these applications.

Here are some:

All-optical computation. Personally, I think this has more potential than quantum computing, as it has already been shown to be useful for quickly processing neural networks (and other algorithms involving matrix multiplication and nonlinear functions). These on-chip systems are made in the same labs (and same people) as measurement-based linear quantum computing. Designing systems capable of operating faster than semi-conductor clock speeds, lowering the minimum power-per operation using light, and increasing parallelization will probably get us very far without needing to change algorithmic architectures.

Quantum simulation. Richard Feynman's original dream of "quantum computers" are now what are referred to as "quantum analog simulators." Nature acts like nature. It can be hard to compute analytically or digitally how a Hydrogen atom behaves, but using a system with a similar Hamiltonian can "do the math for you." Optical lattices (which are sometimes used for quantum computing of ions) can be used for these quantum simulators. It is very difficult to do calculations of molecules using fundamental physics and chemistry is full of heuristics to deal with these difficulties.

Quantum state reconstruction. A usually unmentioned open problem in quantum information and computing is how to reconstruct high qbit entangled states. Even if quantum computing doesn't work out, advances made in these open questions might be helpful in the future (for, for instance, key distribution protocols and information theory).

Quantum Communication. Quantum Key distribution is probably the only working practical application created so far from quantum information. It allows information to be transferred safely without the possibility of eavesdroppers. High-fidelity photon gate operations (created for quantum computers) could allow for efficient quantum repeaters, which could extend the maximum distance that can be traveled.

Extra Fun Things. Personally, I think the most interesting thing is answering if the brain is a quantum computer. The possiblity of the brain being a quantum computer has been eye-rolled by many physicists for the last decade, dismissing the high temperatures the brain to destroy coherence, but highly reputable (and commendable) physicists have recently challenged this notion. One discussing how nuclear spins could be the mediator of quantum information, another discussing how experiments could be carried to investigate if axons are operating as waveguides.

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Perform and checking basic quantum-mechanic experiments Before the IBM and alibaba quantum cloud computers, you would need an expensive lab to do simple CHSH or GHZ experiments. Of course the qubits in the IBM computer are not loophole free but many institutes and also collegeschools could not have better experiment facilities purchased within their physics budget. So basic quantum mechanic experiments can be done very easily.

Quantum programming tools and experiments Furthermore basic research in programming quantum computer tools like compilers and mapping algorithms can now be tested on real machines

This has lead to 113 papers with real and tested quantum algorithms for the ibm computer alone and many more in general. qc papers

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